1. Field of the Invention
The invention relates to a noise reducing apparatus, particularly to a noise reducing apparatus of the type comprising a lens-type hollow body for converging incident noises from a sound source.
2. Related Art Statement
Hitherto, a noise reducing apparatus of the type comprising the above-described lens-type hollow body has been well known as disclosed in Japanese Utility Model Application Publication No. 24,716/83. Such a noise reducing apparatus, as shown in FIG. 5, comprises a lens-type hollow body 2 consisting of a plurality of inclined hollow tubular passages 1 having different lengths, and the lens-type hollow body 2 has a convex lens-shaped cross section having a plane incident surface 2a opened towards a sound source and a hyperbolic curved emission surface 2b on the opposite side. Such a convex lens-type hollow body 2 is arranged in a sound field such that parallel waves A from the noise source enter to the openings 1a at the incident surface side of the hollow tubular passages directed to the noise source, so that the sound waves passing through the hollow tubular passages 1 of the lens-type hollow body 2 are focused, behind the lenstype hollow body 2. Therefore, it is possible to absorb and reduce noises by arranging a sound absorbing material 3 at a focus F.
However, when noises are generated from an approximate actual sound source such as a vehicle engine, radiator, muffler, pump and the like, the sound waves spherically emit from the spot sound source. It has been confirmed from experiments that when the above-described conventional convex lens-type hollow body 2 is arranged in a sound field where the noises are emitted spherically from the spot sound source, as shown in FIG. 6, sound waves passed through the hollow tubular passages of the convex lens-type hollow body 2 are emitted from an emission surface 2b as shown by an arrow B and cannot be focused